The major focus of this Superfund Program remains the development and validation of in vivo and in vitro bioassays which can be used for risk assessment of oily and wood-preserving wastes. One goal has been to develop and validate a variety of diverse chemical class-specific bioassays that can be utilized for risk assessment of complex toxic mixtures. To achieve this aim, the project has utilized both model compounds and reconstituted mixtures to determine relative potencies of individual congeners and the additive or non-additive interactions which are observed for mixtures. In some studies, the toxic and/or biochemical effects of mixtures obtained from specific hazardous waste sites have also been determined. Progress is reported in determining relative potencies or toxic equivalency factors (TEFs) for some of the most toxic individual compounds that have been identified, and this approach is now being utilized for risk assessment by regulatory agencies worldwide.